This invention relates to crop strippers for harvesting crops from standing plants, and in particular to the rotors of such strippers used for detaching the crops from the plants, and to the toothing of the rotors which perform this function.
Crop strippers are best known for stripping grain from the heads of cereal crops such as wheat, and feed from herbage crops such as grass, but they can also be arranged to harvest other grain or seed crops, or to strip other parts of plants, such as leaves, young shoots or blossom from the stems of crops.
The rotor of a crop stripper has stripper teeth projecting from its periphery, arranged in a series of transversely extended rows or combs, each row usually being made up of a number of similar comb plates, butted end to end. Although many different tooth forms have been proposed in the literature, in practice the customary tooth profile comprises a radially outwardly tapering outer portion extending from a radially inner portion in which the spacing between adjacent teeth increases to form a bulbous root opening having a maximum width greater than the width of a radially outer neck of the opening at the junction with the tapering outer portions of the teeth.
In the operation of the-crop stripper, the rotor is driven as the machine advances through the standing crop, the lower periphery of the rotor turning in the direction of advance, and the plant stems are trapped in the spaces between the projecting teeth. As the stems are drawn through the teeth, the grain or other crop is stripped from the stems.
The efficiency of operation is dependent, to some extent at least, on the ability of the stripper teeth to minimise the amount of unwanted material that is taken up in addition to the crop parts being harvested. For this reason, it has been customary to make the comb plates of a thermoplastic material, which causes little, if any, damage to the crop stems that will detach them. The comb plates are thick enough to be relatively stiff yet they have a sufficient degree of elasticity to avoid permanent deformation if a tooth strikes an obstacle. Because the plastics is a relatively easily abraded material, however, the teeth wear very rapidly in some crops and require frequent replacement. In order to avoid this disadvantage, it has been proposed to protect the inner regions of the tooth profile by placing auxiliary comb plates of shorter metal teeth immediately in front of the plastics teeth (WO92/08339 or U.S. Pat. No. 5,389,038). This is only a partial solution, however, as the radially outer regions of the plastics teeth remain unprotected.
Full-length metal teeth have also been proposed, but they have not been widely employed in practice The comb plates can be stamped or cut from metal sheet and in one proposed form of metal toothing disclosed in WO86/01972, the tapering outer portion of each tooth extending from the neck of the inner portion is provided with radiused or folded edges or flanges. These are intended to give a scooping action if the edges are directed forwards, or to have a squeezing action on the crop passing between them if they are directed rearwards. Because the flanged edges increase the stiffness, the flexure of such teeth under load must be concentrated in the relatively short inner portions where the bulbous openings are formed. If the teeth strike an obstacle, the flexure of these portions may be so great as to deform them plastically. Alternatively, high levels of stress generated in the stiffened outer portions can cause buckling of the flanged edges and so permanently deform the outer portions.
In another proposed form of metal toothing, in WO96/04776, a rearwardly directed flange around substantially the entire periphery of each tooth gives the tooth a rounded edge, with the aim of providing a less aggressive action than a plain stamped edge would give while having a substantially longer working life than plastics teeth. In easily detached crops the gentler effect of this toothing can strip the crop to be harvested while minimising the detachment and uptake of other parts of the plants.
The toothing proposed in WO96/04776 is made of a relatively thin spring grade steel to give the required degree of elasticity with ease of manufacture, and it will have significant flexibility despite the flanging of its edges. The resistance of the crop stems as the stripper teeth move can therefore cause excessive flexure, reducing the efficiency of operation and in order to control the flexure of the teeth it has also been proposed to provide them with a backing of shorter and thicker plastic teeth. These plastic teeth extend outwards beyond the necks of the metal tooth inner portions but terminate some distance from the tips of the outer tapered portions of the teeth, so they are shielded from abrasion. Despite the support provided by the plastic teeth, it is found that the metal toothing is susceptible to cracking, in particular at the transition between the inner and outer portions of a tooth, which can lead to premature failure by fracture. It is proposed in WO96/04776 that the flanging can be interrupted at the angular transitions between the bulbous openings and the tapering teeth to reduce stress concentrations there, but there is a risk that this measure concentrates any flexure under load at that point and lowers the limit at which plastic deformation of the teeth will occur.
It is an object of the present invention to at least ameliorate some of the problems of the prior art metal toothing.